A field-effect transistor (also referred to as a “FET”) is a conventional semiconductor device employable in switch-mode power supplies for use in data processing and telecommunication systems as well as for other applications that use conditioned power for operation of sensitive electronic circuits. The field-effect transistors have almost universally replaced bipolar transistors previously used for inverters (a type or portion of a power supply) as well as the p-n and Schottky diodes used for rectification. The proliferation of the field-effect transistors has, at least in part, been driven by the need to provide highly efficient power supplies with low dc output voltages such as five volts or less at higher current levels. The broad acceptance of the field-effect transistors for the aforementioned applications is a consequence of the low forward voltage drop and fast switching speed as well as the low power consumption employed to enable or disable conduction thereof. As a result, the use of the field-effect transistors has contributed to compact and efficient power supplies that can be produced at low cost.
As the loads for the power supplies are generally designed with integrated circuits employing shrinking feature sizes for the circuit elements, a need is continually evolving for new designs with lower output voltages (e.g., one volt or less) at higher current levels (e.g., 50 to 100 amperes or more). Present switch-mode power supplies providing input-output circuit isolation (via a transformer) and employing silicon-based field-effect transistors as synchronous rectifiers therein and designed with best current practice are usually operable with switching frequencies only up to several hundred kilohertz (“kHz”) due, at least in part, to the slower switching speeds of the silicon-based field-effect transistors. To accommodate continuing developments in integrated circuit technology, however, power supply switching frequencies above one megahertz (“MHz”) are desirable to reduce the size of magnetic devices and the capacitive filtering elements of the power supply without compromising the power conversion efficiency. In accordance therewith, field-effect transistors with previously unrealizable characteristics are not only being requested, but are necessary to satisfy the conditions.
To provide increased power handling capabilities, transistors with a larger effective area have been developed. However, as the area of a transistor increases, the transistor may become less suitable for high frequency operations that, typically, require a small source to drain distance so that the carrier transit times are limited. One technique for increasing the area of a transistor while still providing for high frequency operations is to use a plurality of transistor cells that are connected in parallel. Such a configuration includes a plurality of elongated gate “fingers” which control the flow of current through each of the plurality of unit cells. Thus, the source to drain distance of each cell may be kept relatively small while still providing a transistor with increased power handling capability.
A material of choice for high performance field-effect transistors is a semiconductor with high electron mobility, and wide band gap for high breakdown voltage that is capable of being processed with conventional equipment and methods not substantially different from those already developed for silicon and present generations of compound semiconductors. A particularly desirable material is the compound semiconductor known as gallium nitride (“GaN”), which has been used for integrated circuits operable at frequencies well above one gigahertz, and which has been used to produce power field-effect transistors with high performance characteristics. The GaN-based FET device is capable of maximizing electron mobility by forming a quantum well at the heterojunction interface between a AlGaN barrier layer, which has a large band gap, and the GaN layer, which has a narrower band gap. As a result, electrons are trapped in the quantum well. The trapped electrons are represented by a two-dimensional electron gas in the undoped GaN layer. The amount of current is controlled by applying voltage to the gate electrode, which is in Schottky contact with the semiconductors so that electrons flow along the channel between the source electrode and the drain electrode.
As the market for GaN-based FETs continues to grow, many improvements remain desirable to enhance various operating characteristics such as the breakdown voltage Vbr, on-resistance and the cut-off frequency.